diff --git a/executable_semantics/ast/declaration.h b/executable_semantics/ast/declaration.h
index 13c706456f25..3918edd7f47b 100644
--- a/executable_semantics/ast/declaration.h
+++ b/executable_semantics/ast/declaration.h
@@ -127,10 +127,11 @@ struct StructDeclaration {
 struct ChoiceDeclaration {
   int line_num;
   std::string name;
-  std::list<std::pair<std::string, Expression*>> alternatives;
+  std::list<std::pair<std::string, const Expression*>> alternatives;
 
-  ChoiceDeclaration(int line_num, std::string name,
-                    std::list<std::pair<std::string, Expression*>> alternatives)
+  ChoiceDeclaration(
+      int line_num, std::string name,
+      std::list<std::pair<std::string, const Expression*>> alternatives)
       : line_num(line_num), name(name), alternatives(alternatives) {}
 
   void Print() const;
@@ -143,8 +144,8 @@ struct ChoiceDeclaration {
 // Global variable definition implements the Declaration concept.
 class VariableDeclaration {
  public:
-  VariableDeclaration(int source_location, std::string name, Expression* type,
-                      Expression* initializer)
+  VariableDeclaration(int source_location, std::string name,
+                      const Expression* type, const Expression* initializer)
       : source_location(source_location),
         name(name),
         type(type),
@@ -159,8 +160,8 @@ class VariableDeclaration {
  private:
   int source_location;
   std::string name;
-  Expression* type;
-  Expression* initializer;
+  const Expression* type;
+  const Expression* initializer;
 };
 
 }  // namespace Carbon
diff --git a/executable_semantics/ast/expression.cpp b/executable_semantics/ast/expression.cpp
index a9c918cffbd5..6303a107e35b 100644
--- a/executable_semantics/ast/expression.cpp
+++ b/executable_semantics/ast/expression.cpp
@@ -8,28 +8,28 @@
 
 namespace Carbon {
 
-auto MakeTypeType(int line_num) -> Expression* {
+auto MakeTypeType(int line_num) -> const Expression* {
   auto* t = new Expression();
   t->tag = ExpressionKind::TypeT;
   t->line_num = line_num;
   return t;
 }
 
-auto MakeIntType(int line_num) -> Expression* {
+auto MakeIntType(int line_num) -> const Expression* {
   auto* t = new Expression();
   t->tag = ExpressionKind::IntT;
   t->line_num = line_num;
   return t;
 }
 
-auto MakeBoolType(int line_num) -> Expression* {
+auto MakeBoolType(int line_num) -> const Expression* {
   auto* t = new Expression();
   t->tag = ExpressionKind::BoolT;
   t->line_num = line_num;
   return t;
 }
 
-auto MakeAutoType(int line_num) -> Expression* {
+auto MakeAutoType(int line_num) -> const Expression* {
   auto* t = new Expression();
   t->tag = ExpressionKind::AutoT;
   t->line_num = line_num;
@@ -37,15 +37,15 @@ auto MakeAutoType(int line_num) -> Expression* {
 }
 
 // Returns a Continuation type AST node at the given source location.
-auto MakeContinuationType(int line_num) -> Expression* {
+auto MakeContinuationType(int line_num) -> const Expression* {
   auto* type = new Expression();
   type->tag = ExpressionKind::ContinuationT;
   type->line_num = line_num;
   return type;
 }
 
-auto MakeFunType(int line_num, Expression* param, Expression* ret)
-    -> Expression* {
+auto MakeFunType(int line_num, const Expression* param, const Expression* ret)
+    -> const Expression* {
   auto* t = new Expression();
   t->tag = ExpressionKind::FunctionT;
   t->line_num = line_num;
@@ -54,7 +54,7 @@ auto MakeFunType(int line_num, Expression* param, Expression* ret)
   return t;
 }
 
-auto MakeVar(int line_num, std::string var) -> Expression* {
+auto MakeVar(int line_num, std::string var) -> const Expression* {
   auto* v = new Expression();
   v->line_num = line_num;
   v->tag = ExpressionKind::Variable;
@@ -62,8 +62,8 @@ auto MakeVar(int line_num, std::string var) -> Expression* {
   return v;
 }
 
-auto MakeVarPat(int line_num, std::string var, Expression* type)
-    -> Expression* {
+auto MakeVarPat(int line_num, std::string var, const Expression* type)
+    -> const Expression* {
   auto* v = new Expression();
   v->line_num = line_num;
   v->tag = ExpressionKind::PatternVariable;
@@ -72,7 +72,7 @@ auto MakeVarPat(int line_num, std::string var, Expression* type)
   return v;
 }
 
-auto MakeInt(int line_num, int i) -> Expression* {
+auto MakeInt(int line_num, int i) -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::Integer;
@@ -80,7 +80,7 @@ auto MakeInt(int line_num, int i) -> Expression* {
   return e;
 }
 
-auto MakeBool(int line_num, bool b) -> Expression* {
+auto MakeBool(int line_num, bool b) -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::Boolean;
@@ -88,8 +88,8 @@ auto MakeBool(int line_num, bool b) -> Expression* {
   return e;
 }
 
-auto MakeOp(int line_num, enum Operator op, std::vector<Expression*>* args)
-    -> Expression* {
+auto MakeOp(int line_num, enum Operator op,
+            std::vector<const Expression*>* args) -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::PrimitiveOp;
@@ -98,31 +98,33 @@ auto MakeOp(int line_num, enum Operator op, std::vector<Expression*>* args)
   return e;
 }
 
-auto MakeUnOp(int line_num, enum Operator op, Expression* arg) -> Expression* {
+auto MakeUnOp(int line_num, enum Operator op, const Expression* arg)
+    -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::PrimitiveOp;
   e->u.primitive_op.op = op;
-  auto* args = new std::vector<Expression*>();
+  auto* args = new std::vector<const Expression*>();
   args->push_back(arg);
   e->u.primitive_op.arguments = args;
   return e;
 }
 
-auto MakeBinOp(int line_num, enum Operator op, Expression* arg1,
-               Expression* arg2) -> Expression* {
+auto MakeBinOp(int line_num, enum Operator op, const Expression* arg1,
+               const Expression* arg2) -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::PrimitiveOp;
   e->u.primitive_op.op = op;
-  auto* args = new std::vector<Expression*>();
+  auto* args = new std::vector<const Expression*>();
   args->push_back(arg1);
   args->push_back(arg2);
   e->u.primitive_op.arguments = args;
   return e;
 }
 
-auto MakeCall(int line_num, Expression* fun, Expression* arg) -> Expression* {
+auto MakeCall(int line_num, const Expression* fun, const Expression* arg)
+    -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::Call;
@@ -131,8 +133,8 @@ auto MakeCall(int line_num, Expression* fun, Expression* arg) -> Expression* {
   return e;
 }
 
-auto MakeGetField(int line_num, Expression* exp, std::string field)
-    -> Expression* {
+auto MakeGetField(int line_num, const Expression* exp, std::string field)
+    -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::GetField;
@@ -142,8 +144,8 @@ auto MakeGetField(int line_num, Expression* exp, std::string field)
 }
 
 auto MakeTuple(int line_num,
-               std::vector<std::pair<std::string, Expression*>>* args)
-    -> Expression* {
+               std::vector<std::pair<std::string, const Expression*>>* args)
+    -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::Tuple;
@@ -161,16 +163,17 @@ auto MakeTuple(int line_num,
 // Create an AST node for an empty tuple.
 // TODO(geoffromer): remove this and rewrite its callers to use
 // `MakeTuple(line_num, {})`, once that works.
-auto MakeUnit(int line_num) -> Expression* {
+auto MakeUnit(int line_num) -> const Expression* {
   auto* unit = new Expression();
   unit->line_num = line_num;
   unit->tag = ExpressionKind::Tuple;
-  auto* args = new std::vector<std::pair<std::string, Expression*>>();
+  auto* args = new std::vector<std::pair<std::string, const Expression*>>();
   unit->u.tuple.fields = args;
   return unit;
 }
 
-auto MakeIndex(int line_num, Expression* exp, Expression* i) -> Expression* {
+auto MakeIndex(int line_num, const Expression* exp, const Expression* i)
+    -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
   e->tag = ExpressionKind::Index;
@@ -206,7 +209,7 @@ static void PrintOp(Operator op) {
 }
 
 static void PrintFields(
-    std::vector<std::pair<std::string, Expression*>>* fields) {
+    std::vector<std::pair<std::string, const Expression*>>* fields) {
   int i = 0;
   for (auto iter = fields->begin(); iter != fields->end(); ++iter, ++i) {
     if (i != 0) {
diff --git a/executable_semantics/ast/expression.h b/executable_semantics/ast/expression.h
index 99ff25f2c514..d9778ec0cafc 100644
--- a/executable_semantics/ast/expression.h
+++ b/executable_semantics/ast/expression.h
@@ -47,73 +47,77 @@ struct Expression {
     } variable;
 
     struct {
-      Expression* aggregate;
+      const Expression* aggregate;
       std::string* field;
     } get_field;
 
     struct {
-      Expression* aggregate;
-      Expression* offset;
+      const Expression* aggregate;
+      const Expression* offset;
     } index;
 
     struct {
       std::string* name;
-      Expression* type;
+      const Expression* type;
     } pattern_variable;
 
     int integer;
     bool boolean;
 
     struct {
-      std::vector<std::pair<std::string, Expression*>>* fields;
+      std::vector<std::pair<std::string, const Expression*>>* fields;
     } tuple;
 
     struct {
       Operator op;
-      std::vector<Expression*>* arguments;
+      std::vector<const Expression*>* arguments;
     } primitive_op;
 
     struct {
-      Expression* function;
-      Expression* argument;
+      const Expression* function;
+      const Expression* argument;
     } call;
 
     struct {
-      Expression* parameter;
-      Expression* return_type;
+      const Expression* parameter;
+      const Expression* return_type;
     } function_type;
 
   } u;
 };
 
-auto MakeVar(int line_num, std::string var) -> Expression*;
-auto MakeVarPat(int line_num, std::string var, Expression* type) -> Expression*;
-auto MakeInt(int line_num, int i) -> Expression*;
-auto MakeBool(int line_num, bool b) -> Expression*;
-auto MakeOp(int line_num, Operator op, std::vector<Expression*>* args)
-    -> Expression*;
-auto MakeUnOp(int line_num, enum Operator op, Expression* arg) -> Expression*;
-auto MakeBinOp(int line_num, enum Operator op, Expression* arg1,
-               Expression* arg2) -> Expression*;
-auto MakeCall(int line_num, Expression* fun, Expression* arg) -> Expression*;
-auto MakeGetField(int line_num, Expression* exp, std::string field)
-    -> Expression*;
+auto MakeVar(int line_num, std::string var) -> const Expression*;
+auto MakeVarPat(int line_num, std::string var, const Expression* type)
+    -> const Expression*;
+auto MakeInt(int line_num, int i) -> const Expression*;
+auto MakeBool(int line_num, bool b) -> const Expression*;
+auto MakeOp(int line_num, Operator op, std::vector<const Expression*>* args)
+    -> const Expression*;
+auto MakeUnOp(int line_num, enum Operator op, const Expression* arg)
+    -> const Expression*;
+auto MakeBinOp(int line_num, enum Operator op, const Expression* arg1,
+               const Expression* arg2) -> const Expression*;
+auto MakeCall(int line_num, const Expression* fun, const Expression* arg)
+    -> const Expression*;
+auto MakeGetField(int line_num, const Expression* exp, std::string field)
+    -> const Expression*;
 auto MakeTuple(int line_num,
-               std::vector<std::pair<std::string, Expression*>>* args)
-    -> Expression*;
+               std::vector<std::pair<std::string, const Expression*>>* args)
+    -> const Expression*;
 // Create an AST node for an empty tuple.
-auto MakeUnit(int line_num) -> Expression*;
-auto MakeIndex(int line_num, Expression* exp, Expression* i) -> Expression*;
+auto MakeUnit(int line_num) -> const Expression*;
+auto MakeIndex(int line_num, const Expression* exp, const Expression* i)
+    -> const Expression*;
 
-auto MakeTypeType(int line_num) -> Expression*;
-auto MakeIntType(int line_num) -> Expression*;
-auto MakeBoolType(int line_num) -> Expression*;
-auto MakeFunType(int line_num, Expression* param, Expression* ret)
-    -> Expression*;
-auto MakeAutoType(int line_num) -> Expression*;
+auto MakeTypeType(int line_num) -> const Expression*;
+auto MakeIntType(int line_num) -> const Expression*;
+auto MakeBoolType(int line_num) -> const Expression*;
+auto MakeFunType(int line_num, const Expression* param, const Expression* ret)
+    -> const Expression*;
+auto MakeAutoType(int line_num) -> const Expression*;
 // Returns a Continuation type AST node at the given source location,
 // which is the type of a continuation value.
-auto MakeContinuationType(int line_num) -> Expression*;
+auto MakeContinuationType(int line_num) -> const Expression*;
 
 void PrintExp(const Expression* exp);
 
diff --git a/executable_semantics/ast/function_definition.cpp b/executable_semantics/ast/function_definition.cpp
index b5484898564a..47b34daafd75 100644
--- a/executable_semantics/ast/function_definition.cpp
+++ b/executable_semantics/ast/function_definition.cpp
@@ -8,8 +8,8 @@
 
 namespace Carbon {
 
-auto MakeFunDef(int line_num, std::string name, Expression* ret_type,
-                Expression* param_pattern, Statement* body)
+auto MakeFunDef(int line_num, std::string name, const Expression* ret_type,
+                const Expression* param_pattern, const Statement* body)
     -> struct FunctionDefinition* {
   auto* f = new struct FunctionDefinition();
   f->line_num = line_num;
diff --git a/executable_semantics/ast/function_definition.h b/executable_semantics/ast/function_definition.h
index 001ba5e869e0..377e3c951d03 100644
--- a/executable_semantics/ast/function_definition.h
+++ b/executable_semantics/ast/function_definition.h
@@ -13,13 +13,14 @@ namespace Carbon {
 struct FunctionDefinition {
   int line_num;
   std::string name;
-  Expression* param_pattern;
-  Expression* return_type;
-  Statement* body;
+  const Expression* param_pattern;
+  const Expression* return_type;
+  const Statement* body;
 };
 
-auto MakeFunDef(int line_num, std::string name, Expression* ret_type,
-                Expression* param, Statement* body) -> FunctionDefinition*;
+auto MakeFunDef(int line_num, std::string name, const Expression* ret_type,
+                const Expression* param, const Statement* body)
+    -> FunctionDefinition*;
 void PrintFunDef(const FunctionDefinition*);
 void PrintFunDefDepth(const FunctionDefinition*, int);
 
diff --git a/executable_semantics/ast/member.cpp b/executable_semantics/ast/member.cpp
index 7770bc07904b..5504c39162ef 100644
--- a/executable_semantics/ast/member.cpp
+++ b/executable_semantics/ast/member.cpp
@@ -8,7 +8,8 @@
 
 namespace Carbon {
 
-auto MakeField(int line_num, std::string name, Expression* type) -> Member* {
+auto MakeField(int line_num, std::string name, const Expression* type)
+    -> Member* {
   auto m = new Member();
   m->line_num = line_num;
   m->tag = MemberKind::FieldMember;
diff --git a/executable_semantics/ast/member.h b/executable_semantics/ast/member.h
index 7db721d7af31..68088e10113c 100644
--- a/executable_semantics/ast/member.h
+++ b/executable_semantics/ast/member.h
@@ -19,12 +19,13 @@ struct Member {
   union {
     struct {
       std::string* name;
-      Expression* type;
+      const Expression* type;
     } field;
   } u;
 };
 
-auto MakeField(int line_num, std::string name, Expression* type) -> Member*;
+auto MakeField(int line_num, std::string name, const Expression* type)
+    -> Member*;
 
 void PrintMember(Member* m);
 
diff --git a/executable_semantics/ast/statement.cpp b/executable_semantics/ast/statement.cpp
index 3fe133646b1b..1ff9f73c3568 100644
--- a/executable_semantics/ast/statement.cpp
+++ b/executable_semantics/ast/statement.cpp
@@ -8,7 +8,7 @@
 
 namespace Carbon {
 
-auto MakeExpStmt(int line_num, Expression* exp) -> Statement* {
+auto MakeExpStmt(int line_num, const Expression* exp) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::ExpressionStatement;
@@ -16,7 +16,8 @@ auto MakeExpStmt(int line_num, Expression* exp) -> Statement* {
   return s;
 }
 
-auto MakeAssign(int line_num, Expression* lhs, Expression* rhs) -> Statement* {
+auto MakeAssign(int line_num, const Expression* lhs, const Expression* rhs)
+    -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Assign;
@@ -25,7 +26,8 @@ auto MakeAssign(int line_num, Expression* lhs, Expression* rhs) -> Statement* {
   return s;
 }
 
-auto MakeVarDef(int line_num, Expression* pat, Expression* init) -> Statement* {
+auto MakeVarDef(int line_num, const Expression* pat, const Expression* init)
+    -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::VariableDefinition;
@@ -34,8 +36,8 @@ auto MakeVarDef(int line_num, Expression* pat, Expression* init) -> Statement* {
   return s;
 }
 
-auto MakeIf(int line_num, Expression* cond, Statement* then_stmt,
-            Statement* else_stmt) -> Statement* {
+auto MakeIf(int line_num, const Expression* cond, const Statement* then_stmt,
+            const Statement* else_stmt) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::If;
@@ -45,7 +47,8 @@ auto MakeIf(int line_num, Expression* cond, Statement* then_stmt,
   return s;
 }
 
-auto MakeWhile(int line_num, Expression* cond, Statement* body) -> Statement* {
+auto MakeWhile(int line_num, const Expression* cond, const Statement* body)
+    -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::While;
@@ -54,21 +57,21 @@ auto MakeWhile(int line_num, Expression* cond, Statement* body) -> Statement* {
   return s;
 }
 
-auto MakeBreak(int line_num) -> Statement* {
+auto MakeBreak(int line_num) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Break;
   return s;
 }
 
-auto MakeContinue(int line_num) -> Statement* {
+auto MakeContinue(int line_num) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Continue;
   return s;
 }
 
-auto MakeReturn(int line_num, Expression* e) -> Statement* {
+auto MakeReturn(int line_num, const Expression* e) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Return;
@@ -76,7 +79,8 @@ auto MakeReturn(int line_num, Expression* e) -> Statement* {
   return s;
 }
 
-auto MakeSeq(int line_num, Statement* s1, Statement* s2) -> Statement* {
+auto MakeSeq(int line_num, const Statement* s1, const Statement* s2)
+    -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Sequence;
@@ -85,7 +89,7 @@ auto MakeSeq(int line_num, Statement* s1, Statement* s2) -> Statement* {
   return s;
 }
 
-auto MakeBlock(int line_num, Statement* stmt) -> Statement* {
+auto MakeBlock(int line_num, const Statement* stmt) -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Block;
@@ -93,9 +97,10 @@ auto MakeBlock(int line_num, Statement* stmt) -> Statement* {
   return s;
 }
 
-auto MakeMatch(int line_num, Expression* exp,
-               std::list<std::pair<Expression*, Statement*>>* clauses)
-    -> Statement* {
+auto MakeMatch(
+    int line_num, const Expression* exp,
+    std::list<std::pair<const Expression*, const Statement*>>* clauses)
+    -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;
   s->tag = StatementKind::Match;
@@ -107,7 +112,7 @@ auto MakeMatch(int line_num, Expression* exp,
 // Returns an AST node for a continuation statement give its line number and
 // parts.
 auto MakeContinuationStatement(int line_num, std::string continuation_variable,
-                               Statement* body) -> Statement* {
+                               const Statement* body) -> const Statement* {
   auto* continuation = new Statement();
   continuation->line_num = line_num;
   continuation->tag = StatementKind::Continuation;
@@ -118,7 +123,7 @@ auto MakeContinuationStatement(int line_num, std::string continuation_variable,
 }
 
 // Returns an AST node for a run statement give its line number and argument.
-auto MakeRun(int line_num, Expression* argument) -> Statement* {
+auto MakeRun(int line_num, const Expression* argument) -> const Statement* {
   auto* run = new Statement();
   run->line_num = line_num;
   run->tag = StatementKind::Run;
@@ -127,14 +132,14 @@ auto MakeRun(int line_num, Expression* argument) -> Statement* {
 }
 
 // Returns an AST node for an await statement give its line number.
-auto MakeAwait(int line_num) -> Statement* {
+auto MakeAwait(int line_num) -> const Statement* {
   auto* await = new Statement();
   await->line_num = line_num;
   await->tag = StatementKind::Await;
   return await;
 }
 
-void PrintStatement(Statement* s, int depth) {
+void PrintStatement(const Statement* s, int depth) {
   if (!s) {
     return;
   }
diff --git a/executable_semantics/ast/statement.h b/executable_semantics/ast/statement.h
index fc88585b6fc9..bebfee54d032 100644
--- a/executable_semantics/ast/statement.h
+++ b/executable_semantics/ast/statement.h
@@ -33,71 +33,76 @@ struct Statement {
   StatementKind tag;
 
   union {
-    Expression* exp;
+    const Expression* exp;
 
     struct {
-      Expression* lhs;
-      Expression* rhs;
+      const Expression* lhs;
+      const Expression* rhs;
     } assign;
 
     struct {
-      Expression* pat;
-      Expression* init;
+      const Expression* pat;
+      const Expression* init;
     } variable_definition;
 
     struct {
-      Expression* cond;
-      Statement* then_stmt;
-      Statement* else_stmt;
+      const Expression* cond;
+      const Statement* then_stmt;
+      const Statement* else_stmt;
     } if_stmt;
 
-    Expression* return_stmt;
+    const Expression* return_stmt;
 
     struct {
-      Statement* stmt;
-      Statement* next;
+      const Statement* stmt;
+      const Statement* next;
     } sequence;
 
     struct {
-      Statement* stmt;
+      const Statement* stmt;
     } block;
 
     struct {
-      Expression* cond;
-      Statement* body;
+      const Expression* cond;
+      const Statement* body;
     } while_stmt;
 
     struct {
-      Expression* exp;
-      std::list<std::pair<Expression*, Statement*>>* clauses;
+      const Expression* exp;
+      std::list<std::pair<const Expression*, const Statement*>>* clauses;
     } match_stmt;
 
     struct {
       std::string* continuation_variable;
-      Statement* body;
+      const Statement* body;
     } continuation;
 
     struct {
-      Expression* argument;
+      const Expression* argument;
     } run;
 
   } u;
 };
 
-auto MakeExpStmt(int line_num, Expression* exp) -> Statement*;
-auto MakeAssign(int line_num, Expression* lhs, Expression* rhs) -> Statement*;
-auto MakeVarDef(int line_num, Expression* pat, Expression* init) -> Statement*;
-auto MakeIf(int line_num, Expression* cond, Statement* then_stmt,
-            Statement* else_stmt) -> Statement*;
-auto MakeReturn(int line_num, Expression* e) -> Statement*;
-auto MakeSeq(int line_num, Statement* s1, Statement* s2) -> Statement*;
-auto MakeBlock(int line_num, Statement* s) -> Statement*;
-auto MakeWhile(int line_num, Expression* cond, Statement* body) -> Statement*;
-auto MakeBreak(int line_num) -> Statement*;
-auto MakeContinue(int line_num) -> Statement*;
-auto MakeMatch(int line_num, Expression* exp,
-               std::list<std::pair<Expression*, Statement*>>* clauses)
-    -> Statement*;
+auto MakeExpStmt(int line_num, const Expression* exp) -> const Statement*;
+auto MakeAssign(int line_num, const Expression* lhs, const Expression* rhs)
+    -> const Statement*;
+auto MakeVarDef(int line_num, const Expression* pat, const Expression* init)
+    -> const Statement*;
+auto MakeIf(int line_num, const Expression* cond, const Statement* then_stmt,
+            const Statement* else_stmt) -> const Statement*;
+auto MakeReturn(int line_num, const Expression* e) -> const Statement*;
+auto MakeSeq(int line_num, const Statement* s1, const Statement* s2)
+    -> const Statement*;
+auto MakeBlock(int line_num, const Statement* s) -> const Statement*;
+auto MakeWhile(int line_num, const Expression* cond, const Statement* body)
+    -> const Statement*;
+auto MakeBreak(int line_num) -> const Statement*;
+auto MakeContinue(int line_num) -> const Statement*;
+auto MakeMatch(
+    int line_num, const Expression* exp,
+    std::list<std::pair<const Expression*, const Statement*>>* clauses)
+    -> const Statement*;
 // Returns an AST node for a continuation statement give its line number and
 // contituent parts.
 //
@@ -105,17 +110,17 @@ auto MakeMatch(int line_num, Expression* exp,
 //       <body>
 //     }
 auto MakeContinuationStatement(int line_num, std::string continuation_variable,
-                               Statement* body) -> Statement*;
+                               const Statement* body) -> const Statement*;
 // Returns an AST node for a run statement give its line number and argument.
 //
 //     __run <argument>;
-auto MakeRun(int line_num, Expression* argument) -> Statement*;
+auto MakeRun(int line_num, const Expression* argument) -> const Statement*;
 // Returns an AST node for an await statement give its line number.
 //
 //     __await;
-auto MakeAwait(int line_num) -> Statement*;
+auto MakeAwait(int line_num) -> const Statement*;
 
-void PrintStatement(Statement*, int);
+void PrintStatement(const Statement*, int);
 
 }  // namespace Carbon
 
diff --git a/executable_semantics/interpreter/action.cpp b/executable_semantics/interpreter/action.cpp
index e35397e7ef92..0c6b349c832b 100644
--- a/executable_semantics/interpreter/action.cpp
+++ b/executable_semantics/interpreter/action.cpp
@@ -59,7 +59,7 @@ void PrintActList(Stack<Action*> ls, std::ostream& out) {
   }
 }
 
-auto MakeExpAct(Expression* e) -> Action* {
+auto MakeExpAct(const Expression* e) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::ExpressionAction;
   act->u.exp = e;
@@ -67,7 +67,7 @@ auto MakeExpAct(Expression* e) -> Action* {
   return act;
 }
 
-auto MakeLvalAct(Expression* e) -> Action* {
+auto MakeLvalAct(const Expression* e) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::LValAction;
   act->u.exp = e;
@@ -75,7 +75,7 @@ auto MakeLvalAct(Expression* e) -> Action* {
   return act;
 }
 
-auto MakeStmtAct(Statement* s) -> Action* {
+auto MakeStmtAct(const Statement* s) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::StatementAction;
   act->u.stmt = s;
diff --git a/executable_semantics/interpreter/action.h b/executable_semantics/interpreter/action.h
index c8eb132f1e96..0cae062ba26e 100644
--- a/executable_semantics/interpreter/action.h
+++ b/executable_semantics/interpreter/action.h
@@ -27,8 +27,8 @@ enum class ActionKind {
 struct Action {
   ActionKind tag;
   union {
-    Expression* exp;  // for LValAction and ExpressionAction
-    Statement* stmt;
+    const Expression* exp;  // for LValAction and ExpressionAction
+    const Statement* stmt;
     const Value* val;  // for finished actions with a value (ValAction)
     Address delete_tmp;
   } u;
@@ -38,9 +38,9 @@ struct Action {
 
 void PrintAct(Action* act, std::ostream& out);
 void PrintActList(Stack<Action*> ls, std::ostream& out);
-auto MakeExpAct(Expression* e) -> Action*;
-auto MakeLvalAct(Expression* e) -> Action*;
-auto MakeStmtAct(Statement* s) -> Action*;
+auto MakeExpAct(const Expression* e) -> Action*;
+auto MakeLvalAct(const Expression* e) -> Action*;
+auto MakeStmtAct(const Statement* s) -> Action*;
 auto MakeValAct(const Value* v) -> Action*;
 auto MakeExpToLvalAct() -> Action*;
 auto MakeDeleteAct(Address a) -> Action*;
diff --git a/executable_semantics/interpreter/interpreter.cpp b/executable_semantics/interpreter/interpreter.cpp
index dbdde1a73377..bcb5cf8ecaaa 100644
--- a/executable_semantics/interpreter/interpreter.cpp
+++ b/executable_semantics/interpreter/interpreter.cpp
@@ -387,7 +387,7 @@ void KillLocals(int line_num, Frame* frame) {
   }
 }
 
-void CreateTuple(Frame* frame, Action* act, Expression* /*exp*/) {
+void CreateTuple(Frame* frame, Action* act, const Expression* /*exp*/) {
   //    { { (v1,...,vn) :: C, E, F} :: S, H}
   // -> { { `(v1,...,vn) :: C, E, F} :: S, H}
   auto elts = new std::vector<std::pair<std::string, Address>>();
@@ -566,7 +566,7 @@ void PatternAssignment(const Value* pat, const Value* val, int line_num) {
 void StepLvalue() {
   Frame* frame = state->stack.Top();
   Action* act = frame->todo.Top();
-  Expression* exp = act->u.exp;
+  const Expression* exp = act->u.exp;
   if (tracing_output) {
     std::cout << "--- step lvalue ";
     PrintExp(exp);
@@ -606,7 +606,7 @@ void StepLvalue() {
     case ExpressionKind::Tuple: {
       //    { {(f1=e1,...) :: C, E, F} :: S, H}
       // -> { {e1 :: (f1=[],...) :: C, E, F} :: S, H}
-      Expression* e1 = (*exp->u.tuple.fields)[0].second;
+      const Expression* e1 = (*exp->u.tuple.fields)[0].second;
       frame->todo.Push(MakeLvalAct(e1));
       act->pos++;
       break;
@@ -634,7 +634,7 @@ void StepLvalue() {
 void StepExp() {
   Frame* frame = state->stack.Top();
   Action* act = frame->todo.Top();
-  Expression* exp = act->u.exp;
+  const Expression* exp = act->u.exp;
   if (tracing_output) {
     std::cout << "--- step exp ";
     PrintExp(exp);
@@ -657,7 +657,7 @@ void StepExp() {
       if (exp->u.tuple.fields->size() > 0) {
         //    { {(f1=e1,...) :: C, E, F} :: S, H}
         // -> { {e1 :: (f1=[],...) :: C, E, F} :: S, H}
-        Expression* e1 = (*exp->u.tuple.fields)[0].second;
+        const Expression* e1 = (*exp->u.tuple.fields)[0].second;
         frame->todo.Push(MakeExpAct(e1));
         act->pos++;
       } else {
@@ -788,7 +788,7 @@ auto IsBlockAct(Action* act) -> bool {
 void StepStmt() {
   Frame* frame = state->stack.Top();
   Action* act = frame->todo.Top();
-  Statement* const stmt = act->u.stmt;
+  const Statement* stmt = act->u.stmt;
   assert(stmt != nullptr && "null statement!");
   if (tracing_output) {
     std::cout << "--- step stmt ";
@@ -1027,7 +1027,7 @@ void HandleValue() {
       break;
     }
     case ActionKind::LValAction: {
-      Expression* exp = act->u.exp;
+      const Expression* exp = act->u.exp;
       switch (exp->tag) {
         case ExpressionKind::GetField: {
           //    { v :: [].f :: C, E, F} :: S, H}
@@ -1066,7 +1066,7 @@ void HandleValue() {
             //    H}
             // -> { { ek+1 :: (f1=v1,..., fk=vk, fk+1=[],...) :: C, E, F} :: S,
             // H}
-            Expression* elt = (*exp->u.tuple.fields)[act->pos].second;
+            const Expression* elt = (*exp->u.tuple.fields)[act->pos].second;
             frame->todo.Pop(1);
             frame->todo.Push(MakeLvalAct(elt));
           } else {
@@ -1083,7 +1083,7 @@ void HandleValue() {
       break;
     }
     case ActionKind::ExpressionAction: {
-      Expression* exp = act->u.exp;
+      const Expression* exp = act->u.exp;
       switch (exp->tag) {
         case ExpressionKind::PatternVariable: {
           auto v =
@@ -1098,7 +1098,7 @@ void HandleValue() {
             //    H}
             // -> { { ek+1 :: (f1=v1,..., fk=vk, fk+1=[],...) :: C, E, F} :: S,
             // H}
-            Expression* elt = (*exp->u.tuple.fields)[act->pos].second;
+            const Expression* elt = (*exp->u.tuple.fields)[act->pos].second;
             frame->todo.Pop(1);
             frame->todo.Push(MakeExpAct(elt));
           } else {
@@ -1153,7 +1153,7 @@ void HandleValue() {
               static_cast<int>(exp->u.primitive_op.arguments->size())) {
             //    { {v :: op(vs,[],e,es) :: C, E, F} :: S, H}
             // -> { {e :: op(vs,v,[],es) :: C, E, F} :: S, H}
-            Expression* arg = (*exp->u.primitive_op.arguments)[act->pos];
+            const Expression* arg = (*exp->u.primitive_op.arguments)[act->pos];
             frame->todo.Pop(1);
             frame->todo.Push(MakeExpAct(arg));
           } else {
@@ -1214,7 +1214,7 @@ void HandleValue() {
       break;
     }
     case ActionKind::StatementAction: {
-      Statement* stmt = act->u.stmt;
+      const Statement* stmt = act->u.stmt;
       switch (stmt->tag) {
         case StatementKind::ExpressionStatement:
           frame->todo.Pop(2);
@@ -1328,7 +1328,8 @@ void HandleValue() {
             if (matches) {  // we have a match, start the body
               auto* new_scope = new Scope(*matches, vars);
               frame->scopes.Push(new_scope);
-              Statement* body_block = MakeBlock(stmt->line_num, c->second);
+              const Statement* body_block =
+                  MakeBlock(stmt->line_num, c->second);
               Action* body_act = MakeStmtAct(body_block);
               body_act->pos = 0;
               frame->todo.Pop(2);
@@ -1440,9 +1441,9 @@ auto InterpProgram(std::list<Declaration>* fs) -> int {
   }
   InitGlobals(fs);
 
-  Expression* arg =
-      MakeTuple(0, new std::vector<std::pair<std::string, Expression*>>());
-  Expression* call_main = MakeCall(0, MakeVar(0, "main"), arg);
+  const Expression* arg = MakeTuple(
+      0, new std::vector<std::pair<std::string, const Expression*>>());
+  const Expression* call_main = MakeCall(0, MakeVar(0, "main"), arg);
   auto todo = Stack(MakeExpAct(call_main));
   auto* scope = new Scope(globals, std::list<std::string>());
   auto* frame = new Frame("top", Stack(scope), todo);
@@ -1466,7 +1467,7 @@ auto InterpProgram(std::list<Declaration>* fs) -> int {
 }
 
 // Interpret an expression at compile-time.
-auto InterpExp(Env values, Expression* e) -> const Value* {
+auto InterpExp(Env values, const Expression* e) -> const Value* {
   auto todo = Stack(MakeExpAct(e));
   auto* scope = new Scope(values, std::list<std::string>());
   auto* frame = new Frame("InterpExp", Stack(scope), todo);
diff --git a/executable_semantics/interpreter/interpreter.h b/executable_semantics/interpreter/interpreter.h
index 470adab9c500..f7b972ee24a5 100644
--- a/executable_semantics/interpreter/interpreter.h
+++ b/executable_semantics/interpreter/interpreter.h
@@ -77,7 +77,7 @@ auto ToInteger(const Value* v) -> int;
 /***** Interpreters *****/
 
 auto InterpProgram(std::list<Declaration>* fs) -> int;
-auto InterpExp(Env values, Expression* e) -> const Value*;
+auto InterpExp(Env values, const Expression* e) -> const Value*;
 
 }  // namespace Carbon
 
diff --git a/executable_semantics/interpreter/typecheck.cpp b/executable_semantics/interpreter/typecheck.cpp
index e2c33ae03fe7..0f3793a5bf18 100644
--- a/executable_semantics/interpreter/typecheck.cpp
+++ b/executable_semantics/interpreter/typecheck.cpp
@@ -40,7 +40,7 @@ void PrintTypeEnv(TypeEnv types, std::ostream& out) {
 }
 
 // Reify type to type expression.
-auto ReifyType(const Value* t, int line_num) -> Expression* {
+auto ReifyType(const Value* t, int line_num) -> const Expression* {
   switch (t->tag) {
     case ValKind::VarTV:
       return MakeVar(0, *t->u.var_type);
@@ -56,7 +56,7 @@ auto ReifyType(const Value* t, int line_num) -> Expression* {
       return MakeFunType(0, ReifyType(t->u.fun_type.param, line_num),
                          ReifyType(t->u.fun_type.ret, line_num));
     case ValKind::TupleV: {
-      auto args = new std::vector<std::pair<std::string, Expression*>>();
+      auto args = new std::vector<std::pair<std::string, const Expression*>>();
       for (auto& field : *t->u.tuple.elts) {
         args->push_back(
             {field.first, ReifyType(state->heap[field.second], line_num)});
@@ -93,7 +93,7 @@ auto ReifyType(const Value* t, int line_num) -> Expression* {
 //    and it is used to implement `auto`, otherwise it is null.
 // context says what kind of position this expression is nested in,
 //    whether it's a position that expects a value, a pattern, or a type.
-auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
+auto TypeCheckExp(const Expression* e, TypeEnv types, Env values,
                   const Value* expected, TCContext context) -> TCResult {
   switch (e->tag) {
     case ExpressionKind::PatternVariable: {
@@ -148,7 +148,8 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
       }
     }
     case ExpressionKind::Tuple: {
-      auto new_args = new std::vector<std::pair<std::string, Expression*>>();
+      auto new_args =
+          new std::vector<std::pair<std::string, const Expression*>>();
       auto arg_types = new std::vector<std::pair<std::string, Address>>();
       auto new_types = types;
       int i = 0;
@@ -184,7 +185,7 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
           // Search for a field
           for (auto& field : *t->u.struct_type.fields) {
             if (*e->u.get_field.field == field.first) {
-              Expression* new_e =
+              const Expression* new_e =
                   MakeGetField(e->line_num, res.exp, *e->u.get_field.field);
               return TCResult(new_e, field.second, res.types);
             }
@@ -192,7 +193,7 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
           // Search for a method
           for (auto& method : *t->u.struct_type.methods) {
             if (*e->u.get_field.field == method.first) {
-              Expression* new_e =
+              const Expression* new_e =
                   MakeGetField(e->line_num, res.exp, *e->u.get_field.field);
               return TCResult(new_e, method.second, res.types);
             }
@@ -217,7 +218,7 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
           for (auto vt = t->u.choice_type.alternatives->begin();
                vt != t->u.choice_type.alternatives->end(); ++vt) {
             if (*e->u.get_field.field == vt->first) {
-              Expression* new_e =
+              const Expression* new_e =
                   MakeGetField(e->line_num, res.exp, *e->u.get_field.field);
               auto fun_ty = MakeFunTypeVal(vt->second, t);
               return TCResult(new_e, fun_ty, res.types);
@@ -252,7 +253,7 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
     case ExpressionKind::Boolean:
       return TCResult(e, MakeBoolTypeVal(), types);
     case ExpressionKind::PrimitiveOp: {
-      auto es = new std::vector<Expression*>();
+      auto es = new std::vector<const Expression*>();
       std::vector<const Value*> ts;
       auto new_types = types;
       for (auto& argument : *e->u.primitive_op.arguments) {
@@ -350,9 +351,10 @@ auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
   }
 }
 
-auto TypecheckCase(const Value* expected, Expression* pat, Statement* body,
-                   TypeEnv types, Env values, const Value*& ret_type)
-    -> std::pair<Expression*, Statement*> {
+auto TypecheckCase(const Value* expected, const Expression* pat,
+                   const Statement* body, TypeEnv types, Env values,
+                   const Value*& ret_type)
+    -> std::pair<const Expression*, const Statement*> {
   auto pat_res =
       TypeCheckExp(pat, types, values, expected, TCContext::PatternContext);
   auto res = TypeCheckStmt(body, pat_res.types, values, ret_type);
@@ -366,7 +368,7 @@ auto TypecheckCase(const Value* expected, Expression* pat, Statement* body,
 // It is the declared return type of the enclosing function definition.
 // If the return type is "auto", then the return type is inferred from
 // the first return statement.
-auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
+auto TypeCheckStmt(const Statement* s, TypeEnv types, Env values,
                    const Value*& ret_type) -> TCStatement {
   if (!s) {
     return TCStatement(s, types);
@@ -376,12 +378,13 @@ auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
       auto res = TypeCheckExp(s->u.match_stmt.exp, types, values, nullptr,
                               TCContext::ValueContext);
       auto res_type = res.type;
-      auto new_clauses = new std::list<std::pair<Expression*, Statement*>>();
+      auto new_clauses =
+          new std::list<std::pair<const Expression*, const Statement*>>();
       for (auto& clause : *s->u.match_stmt.clauses) {
         new_clauses->push_back(TypecheckCase(
             res_type, clause.first, clause.second, types, values, ret_type));
       }
-      Statement* new_s = MakeMatch(s->line_num, res.exp, new_clauses);
+      const Statement* new_s = MakeMatch(s->line_num, res.exp, new_clauses);
       return TCStatement(new_s, types);
     }
     case StatementKind::While: {
@@ -407,7 +410,7 @@ auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
       const Value* rhs_ty = res.type;
       auto lhs_res = TypeCheckExp(s->u.variable_definition.pat, types, values,
                                   rhs_ty, TCContext::PatternContext);
-      Statement* new_s =
+      const Statement* new_s =
           MakeVarDef(s->line_num, s->u.variable_definition.pat, res.exp);
       return TCStatement(new_s, lhs_res.types);
     }
@@ -466,7 +469,7 @@ auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
     case StatementKind::Continuation: {
       TCStatement body_result =
           TypeCheckStmt(s->u.continuation.body, types, values, ret_type);
-      Statement* new_continuation = MakeContinuationStatement(
+      const Statement* new_continuation = MakeContinuationStatement(
           s->line_num, *s->u.continuation.continuation_variable,
           body_result.stmt);
       types.Set(*s->u.continuation.continuation_variable,
@@ -478,7 +481,7 @@ auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
                                               nullptr, TCContext::ValueContext);
       ExpectType(s->line_num, "argument of `run`", MakeContinuationTypeVal(),
                  argument_result.type);
-      Statement* new_run = MakeRun(s->line_num, argument_result.exp);
+      const Statement* new_run = MakeRun(s->line_num, argument_result.exp);
       return TCStatement(new_run, types);
     }
     case StatementKind::Await: {
@@ -488,11 +491,11 @@ auto TypeCheckStmt(Statement* s, TypeEnv types, Env values,
   }  // switch
 }
 
-auto CheckOrEnsureReturn(Statement* stmt, bool void_return, int line_num)
-    -> Statement* {
+auto CheckOrEnsureReturn(const Statement* stmt, bool void_return, int line_num)
+    -> const Statement* {
   if (!stmt) {
     if (void_return) {
-      auto args = new std::vector<std::pair<std::string, Expression*>>();
+      auto args = new std::vector<std::pair<std::string, const Expression*>>();
       return MakeReturn(line_num, MakeTuple(line_num, args));
     } else {
       std::cerr
@@ -503,7 +506,8 @@ auto CheckOrEnsureReturn(Statement* stmt, bool void_return, int line_num)
   }
   switch (stmt->tag) {
     case StatementKind::Match: {
-      auto new_clauses = new std::list<std::pair<Expression*, Statement*>>();
+      auto new_clauses =
+          new std::list<std::pair<const Expression*, const Statement*>>();
       for (auto i = stmt->u.match_stmt.clauses->begin();
            i != stmt->u.match_stmt.clauses->end(); ++i) {
         auto s = CheckOrEnsureReturn(i->second, void_return, stmt->line_num);
@@ -543,7 +547,8 @@ auto CheckOrEnsureReturn(Statement* stmt, bool void_return, int line_num)
     case StatementKind::Continue:
     case StatementKind::VariableDefinition:
       if (void_return) {
-        auto args = new std::vector<std::pair<std::string, Expression*>>();
+        auto args =
+            new std::vector<std::pair<std::string, const Expression*>>();
         return MakeSeq(
             stmt->line_num, stmt,
             MakeReturn(stmt->line_num, MakeTuple(stmt->line_num, args)));
diff --git a/executable_semantics/interpreter/typecheck.h b/executable_semantics/interpreter/typecheck.h
index 38e6846a10d0..17d4bdaaa1dd 100644
--- a/executable_semantics/interpreter/typecheck.h
+++ b/executable_semantics/interpreter/typecheck.h
@@ -21,25 +21,26 @@ void PrintTypeEnv(TypeEnv types);
 enum class TCContext { ValueContext, PatternContext, TypeContext };
 
 struct TCResult {
-  TCResult(Expression* e, const Value* t, TypeEnv types)
+  TCResult(const Expression* e, const Value* t, TypeEnv types)
       : exp(e), type(t), types(types) {}
 
-  Expression* exp;
+  const Expression* exp;
   const Value* type;
   TypeEnv types;
 };
 
 struct TCStatement {
-  TCStatement(Statement* s, TypeEnv types) : stmt(s), types(types) {}
+  TCStatement(const Statement* s, TypeEnv types) : stmt(s), types(types) {}
 
-  Statement* stmt;
+  const Statement* stmt;
   TypeEnv types;
 };
 
-auto TypeCheckExp(Expression* e, TypeEnv types, Env values,
+auto TypeCheckExp(const Expression* e, TypeEnv types, Env values,
                   const Value* expected, TCContext context) -> TCResult;
 
-auto TypeCheckStmt(Statement*, TypeEnv, Env, Value const*&) -> TCStatement;
+auto TypeCheckStmt(const Statement*, TypeEnv, Env, Value const*&)
+    -> TCStatement;
 
 auto TypeCheckFunDef(struct FunctionDefinition*, TypeEnv)
     -> struct FunctionDefinition*;
diff --git a/executable_semantics/interpreter/value.cpp b/executable_semantics/interpreter/value.cpp
index 2a83895c7f7c..b1b2f062fab6 100644
--- a/executable_semantics/interpreter/value.cpp
+++ b/executable_semantics/interpreter/value.cpp
@@ -63,7 +63,7 @@ auto MakeBoolVal(bool b) -> const Value* {
   return v;
 }
 
-auto MakeFunVal(std::string name, const Value* param, Statement* body)
+auto MakeFunVal(std::string name, const Value* param, const Statement* body)
     -> const Value* {
   auto* v = new Value();
   v->tag = ValKind::FunV;
diff --git a/executable_semantics/interpreter/value.h b/executable_semantics/interpreter/value.h
index 03deb45b2fe4..8f3b8014f095 100644
--- a/executable_semantics/interpreter/value.h
+++ b/executable_semantics/interpreter/value.h
@@ -61,7 +61,7 @@ struct Value {
     struct {
       std::string* name;
       const Value* param;
-      Statement* body;
+      const Statement* body;
     } fun;
 
     struct {
@@ -129,7 +129,7 @@ struct Value {
 auto MakeContinuation(std::vector<Frame*> stack) -> Value*;
 auto MakeIntVal(int i) -> const Value*;
 auto MakeBoolVal(bool b) -> const Value*;
-auto MakeFunVal(std::string name, const Value* param, Statement* body)
+auto MakeFunVal(std::string name, const Value* param, const Statement* body)
     -> const Value*;
 auto MakePtrVal(Address addr) -> const Value*;
 auto MakeStructVal(const Value* type, const Value* inits) -> const Value*;
diff --git a/executable_semantics/syntax/paren_contents.cpp b/executable_semantics/syntax/paren_contents.cpp
index 6d6e61ab9d03..080b6c6bb0f9 100644
--- a/executable_semantics/syntax/paren_contents.cpp
+++ b/executable_semantics/syntax/paren_contents.cpp
@@ -6,7 +6,7 @@
 
 namespace Carbon {
 
-Expression* ParenContents::AsExpression(int line_number) const {
+const Expression* ParenContents::AsExpression(int line_number) const {
   if (fields_.size() == 1 && fields_.front().name == "" &&
       has_trailing_comma_ == HasTrailingComma::No) {
     return fields_.front().expression;
@@ -15,8 +15,9 @@ Expression* ParenContents::AsExpression(int line_number) const {
   }
 }
 
-Expression* ParenContents::AsTuple(int line_number) const {
-  auto vec = new std::vector<std::pair<std::string, Carbon::Expression*>>();
+const Expression* ParenContents::AsTuple(int line_number) const {
+  auto vec =
+      new std::vector<std::pair<std::string, const Carbon::Expression*>>();
   for (const FieldInitializer& initializer : fields_) {
     vec->push_back({initializer.name, initializer.expression});
   }
diff --git a/executable_semantics/syntax/paren_contents.h b/executable_semantics/syntax/paren_contents.h
index 8a66c1c234f2..f222ecdb836d 100644
--- a/executable_semantics/syntax/paren_contents.h
+++ b/executable_semantics/syntax/paren_contents.h
@@ -18,7 +18,7 @@ struct FieldInitializer {
   std::string name;
 
   // The expression that initializes the field.
-  Expression* expression;
+  const Expression* expression;
 };
 
 // Represents the syntactic contents of an expression delimited by
@@ -45,11 +45,11 @@ class ParenContents {
   ParenContents& operator=(const ParenContents&) = default;
 
   // Returns the paren expression, interpreted as a tuple.
-  Expression* AsTuple(int line_number) const;
+  const Expression* AsTuple(int line_number) const;
 
   // Returns the paren expression, with no external constraints on what kind
   // of expression it represents.
-  Expression* AsExpression(int line_number) const;
+  const Expression* AsExpression(int line_number) const;
 
  private:
   std::vector<FieldInitializer> fields_;
diff --git a/executable_semantics/syntax/paren_contents_test.cpp b/executable_semantics/syntax/paren_contents_test.cpp
index 9a499d470a4c..cec4b6e467bb 100644
--- a/executable_semantics/syntax/paren_contents_test.cpp
+++ b/executable_semantics/syntax/paren_contents_test.cpp
@@ -11,7 +11,7 @@ namespace {
 
 TEST(ParenContentsTest, EmptyAsExpression) {
   ParenContents contents;
-  Expression* expression = contents.AsExpression(/*line_num=*/1);
+  const Expression* expression = contents.AsExpression(/*line_num=*/1);
   EXPECT_EQ(expression->line_num, 1);
   ASSERT_EQ(expression->tag, ExpressionKind::Tuple);
   EXPECT_EQ(expression->u.tuple.fields->size(), 0);
@@ -19,7 +19,7 @@ TEST(ParenContentsTest, EmptyAsExpression) {
 
 TEST(ParenContentsTest, EmptyAsTuple) {
   ParenContents contents;
-  Expression* tuple = contents.AsTuple(/*line_num=*/1);
+  const Expression* tuple = contents.AsTuple(/*line_num=*/1);
   EXPECT_EQ(tuple->line_num, 1);
   ASSERT_EQ(tuple->tag, ExpressionKind::Tuple);
   EXPECT_EQ(tuple->u.tuple.fields->size(), 0);
@@ -35,7 +35,7 @@ TEST(ParenContentsTest, UnaryNoCommaAsExpression) {
   ParenContents contents({{.expression = MakeInt(/*line_num=*/2, 42)}},
                          ParenContents::HasTrailingComma::No);
 
-  Expression* expression = contents.AsExpression(/*line_num=*/1);
+  const Expression* expression = contents.AsExpression(/*line_num=*/1);
   EXPECT_EQ(expression->line_num, 2);
   ASSERT_EQ(expression->tag, ExpressionKind::Integer);
 }
@@ -44,10 +44,10 @@ TEST(ParenContentsTest, UnaryNoCommaAsTuple) {
   ParenContents contents({{.expression = MakeInt(/*line_num=*/2, 42)}},
                          ParenContents::HasTrailingComma::No);
 
-  Expression* tuple = contents.AsTuple(/*line_num=*/1);
+  const Expression* tuple = contents.AsTuple(/*line_num=*/1);
   EXPECT_EQ(tuple->line_num, 1);
   ASSERT_EQ(tuple->tag, ExpressionKind::Tuple);
-  std::vector<std::pair<std::string, Expression*>> fields =
+  std::vector<std::pair<std::string, const Expression*>> fields =
       *tuple->u.tuple.fields;
   ASSERT_EQ(fields.size(), 1);
   EXPECT_EQ(fields[0].second->tag, ExpressionKind::Integer);
@@ -57,10 +57,10 @@ TEST(ParenContentsTest, UnaryWithCommaAsExpression) {
   ParenContents contents({{.expression = MakeInt(/*line_num=*/2, 42)}},
                          ParenContents::HasTrailingComma::Yes);
 
-  Expression* expression = contents.AsExpression(/*line_num=*/1);
+  const Expression* expression = contents.AsExpression(/*line_num=*/1);
   EXPECT_EQ(expression->line_num, 1);
   ASSERT_EQ(expression->tag, ExpressionKind::Tuple);
-  std::vector<std::pair<std::string, Expression*>> fields =
+  std::vector<std::pair<std::string, const Expression*>> fields =
       *expression->u.tuple.fields;
   ASSERT_EQ(fields.size(), 1);
   EXPECT_EQ(fields[0].second->tag, ExpressionKind::Integer);
@@ -70,10 +70,10 @@ TEST(ParenContentsTest, UnaryWithCommaAsTuple) {
   ParenContents contents({{.expression = MakeInt(/*line_num=*/2, 42)}},
                          ParenContents::HasTrailingComma::Yes);
 
-  Expression* tuple = contents.AsTuple(/*line_num=*/1);
+  const Expression* tuple = contents.AsTuple(/*line_num=*/1);
   EXPECT_EQ(tuple->line_num, 1);
   ASSERT_EQ(tuple->tag, ExpressionKind::Tuple);
-  std::vector<std::pair<std::string, Expression*>> fields =
+  std::vector<std::pair<std::string, const Expression*>> fields =
       *tuple->u.tuple.fields;
   ASSERT_EQ(fields.size(), 1);
   EXPECT_EQ(fields[0].second->tag, ExpressionKind::Integer);
@@ -84,10 +84,10 @@ TEST(ParenContentsTest, BinaryAsExpression) {
                           {.expression = MakeInt(/*line_num=*/3, 42)}},
                          ParenContents::HasTrailingComma::Yes);
 
-  Expression* expression = contents.AsExpression(/*line_num=*/1);
+  const Expression* expression = contents.AsExpression(/*line_num=*/1);
   EXPECT_EQ(expression->line_num, 1);
   ASSERT_EQ(expression->tag, ExpressionKind::Tuple);
-  std::vector<std::pair<std::string, Expression*>> fields =
+  std::vector<std::pair<std::string, const Expression*>> fields =
       *expression->u.tuple.fields;
   ASSERT_EQ(fields.size(), 2);
   EXPECT_EQ(fields[0].second->tag, ExpressionKind::Integer);
@@ -99,10 +99,10 @@ TEST(ParenContentsTest, BinaryAsTuple) {
                           {.expression = MakeInt(/*line_num=*/3, 42)}},
                          ParenContents::HasTrailingComma::Yes);
 
-  Expression* tuple = contents.AsTuple(/*line_num=*/1);
+  const Expression* tuple = contents.AsTuple(/*line_num=*/1);
   EXPECT_EQ(tuple->line_num, 1);
   ASSERT_EQ(tuple->tag, ExpressionKind::Tuple);
-  std::vector<std::pair<std::string, Expression*>> fields =
+  std::vector<std::pair<std::string, const Expression*>> fields =
       *tuple->u.tuple.fields;
   ASSERT_EQ(fields.size(), 2);
   EXPECT_EQ(fields[0].second->tag, ExpressionKind::Integer);
diff --git a/executable_semantics/syntax/parser.ypp b/executable_semantics/syntax/parser.ypp
index c184d42d29e4..9f85afa3f237 100644
--- a/executable_semantics/syntax/parser.ypp
+++ b/executable_semantics/syntax/parser.ypp
@@ -96,24 +96,24 @@ void yy::parser::error(
 %type <Carbon::FunctionDefinition*> function_declaration
 %type <Carbon::FunctionDefinition*> function_definition
 %type <std::list<Carbon::Declaration>*> declaration_list
-%type <Carbon::Statement*> statement
-%type <Carbon::Statement*> optional_else
-%type <Carbon::Statement*> statement_list
-%type <Carbon::Expression*> expression
-%type <Carbon::Expression*> pattern
-%type <Carbon::Expression*> return_type
-%type <Carbon::Expression*> paren_expression
-%type <Carbon::Expression*> tuple
+%type <const Carbon::Statement*> statement
+%type <const Carbon::Statement*> optional_else
+%type <const Carbon::Statement*> statement_list
+%type <const Carbon::Expression*> expression
+%type <const Carbon::Expression*> pattern
+%type <const Carbon::Expression*> return_type
+%type <const Carbon::Expression*> paren_expression
+%type <const Carbon::Expression*> tuple
 %type <Carbon::Member*> variable_declaration
 %type <Carbon::Member*> member
 %type <std::list<Carbon::Member*>*> member_list
 %type <Carbon::FieldInitializer> field_initializer
 %type <Carbon::ParenContents> paren_contents
 %type <std::vector<Carbon::FieldInitializer>> paren_contents_without_trailing_comma
-%type <std::pair<std::string, Carbon::Expression*>*> alternative
-%type <std::list<std::pair<std::string, Carbon::Expression*>>*> alternative_list
-%type <std::pair<Carbon::Expression*, Carbon::Statement*>*> clause
-%type <std::list<std::pair<Carbon::Expression*, Carbon::Statement*>>*> clause_list
+%type <std::pair<std::string, const Carbon::Expression*>*> alternative
+%type <std::list<std::pair<std::string, const Carbon::Expression*>>*> alternative_list
+%type <std::pair<const Carbon::Expression*, const Carbon::Statement*>*> clause
+%type <std::list<std::pair<const Carbon::Expression*, const Carbon::Statement*>>*> clause_list
 %token END_OF_FILE 0
 %token AND
 %token OR
@@ -266,18 +266,18 @@ paren_contents_without_trailing_comma:
 ;
 clause:
   CASE pattern DBLARROW statement
-    { $$ = new std::pair<Carbon::Expression*, Carbon::Statement*>($2, $4); }
+    { $$ = new std::pair<const Carbon::Expression*, const Carbon::Statement*>($2, $4); }
 | DEFAULT DBLARROW statement
     {
       auto vp = Carbon::MakeVarPat(yylineno, "_",
                                    Carbon::MakeAutoType(yylineno));
-      $$ = new std::pair<Carbon::Expression*, Carbon::Statement*>(vp, $3);
+      $$ = new std::pair<const Carbon::Expression*, const Carbon::Statement*>(vp, $3);
     }
 ;
 clause_list:
   // Empty
     {
-      $$ = new std::list<std::pair<Carbon::Expression*, Carbon::Statement*>>();
+      $$ = new std::list<std::pair<const Carbon::Expression*, const Carbon::Statement*>>();
     }
 | clause clause_list
     { $$ = $2; $$->push_front(*$1); }
@@ -326,7 +326,7 @@ return_type:
     {
       $$ = Carbon::MakeTuple(
           yylineno,
-          new std::vector<std::pair<std::string, Carbon::Expression*>>());
+          new std::vector<std::pair<std::string, const Carbon::Expression*>>());
     }
 | ARROW expression
     { $$ = $2; }
@@ -358,21 +358,21 @@ member_list:
 ;
 alternative:
   identifier tuple
-    { $$ = new std::pair<std::string, Carbon::Expression*>($1, $2); }
+    { $$ = new std::pair<std::string, const Carbon::Expression*>($1, $2); }
 | identifier
     {
-      $$ = new std::pair<std::string, Carbon::Expression*>(
+      $$ = new std::pair<std::string, const Carbon::Expression*>(
           $1, Carbon::MakeTuple(
             yylineno,
-            new std::vector<std::pair<std::string, Carbon::Expression*>>()));
+            new std::vector<std::pair<std::string, const Carbon::Expression*>>()));
     }
 ;
 alternative_list:
   // Empty
-    { $$ = new std::list<std::pair<std::string, Carbon::Expression*>>(); }
+    { $$ = new std::list<std::pair<std::string, const Carbon::Expression*>>(); }
 | alternative
     {
-      $$ = new std::list<std::pair<std::string, Carbon::Expression*>>();
+      $$ = new std::list<std::pair<std::string, const Carbon::Expression*>>();
       $$->push_front(*$1);
     }
 | alternative "," alternative_list